Improvement of radiotherapy treatment delivery accuracy using an electronic portal imaging device.

Reliable application of advanced external beam techniques for the treatment of patients with cancer, such as intensity modulated radiotherapy, requires an adequate quality assurance programme for the verification of the dose delivery. Accurate patient positioning is mandatory because of the steep dose gradients outside the tumour volume. Owing to the increased complexity of the treatment planning and delivery techniques, verification of the dose delivery before and during the actual patient treatment is equally important. For this purpose, a quality assurance programme has been established in our clinic that is primarily based on measurements with electronic portal imaging devices. To minimise systematic set-up errors, the patient positioning is measured in the first few treatment fractions and a set-up correction is applied in the subsequent ones. Before the first treatment fraction, portal dose measurements are performed for each treatment field with the electronic portal imaging device to verify that the planned fluence distribution is correctly delivered at the treatment unit. Dosimetric measurements are also performed during patient treatment to derive the actually delivered fluence maps. By combining this information with knowledge on the patient set-up, the delivered 3-D dose distribution to both the tumour and sensitive organs may be assessed. However, for the highest accuracy, exact knowledge on the (internal) patient geometry during treatment, e.g. using a cone-beam CT, is required.